A generic valve is known from EP-A-0 893 607. This known valve is a magnetically operated drain valve in which, between a load pressure port (P) and a drain port (T) in the lifting module of a forklift, a seat closing element is assigned to the main valve seat and in the closing direction can be pressurized to a variable difference between the drain pressure and the control pressure derived from the load pressure. A pilot valve can be actuated by a magnet means provided with a pilot piston for the control pressure. The main valve formed by the main valve seat and the seat closing element is assigned a pressure compensator with a seat valve sealing function. With the main valve, the seat valve forms a two-way flow regulator independent of the load pressure and leak-proof under the load pressure in the closing position of the main valve.
This known approach discloses a structurally simple, magnetically operated drain valve of compact size, with which it is possible to implement a ramp function independently of the load pressure. A ramp function is defined as the possibility of controlling the flow amount depending on lift and independently of pressure. However, the known solution for lowering the load in hydraulic lifting devices does not meet the high demands as desired, specifically achieving a high no-load lowering speed with little leakage and a precise metering of this lowering speed.
Control devices for hydraulically operating lifting means are commercially available, and use directly controlled valves not suitable for high volumetric flow due to the design, so that in general pilot-controlled valves are preferred. In barometrically pilot-controlled valves, an independent pressure supply making available the required pressure for adjusting the main piston is necessary. This pressure is generally 10 to 20 bars, and is often produced by an external supply, for example, the feed pump of the traveling mechanism, in a forklift with an internal combustion engine. In lifts with an electric drive, there is no external supply so that the required control pressure can only be taken from the load pressure. When lowering at no load, the available control pressure can then drop to approximately 2 bars with the result that in no-load lowering the lowering process is hampered.